An operation of specific electronic circuits may vary together with variations of the temperature of the electronic circuit. Transistors and diodes junctions have a current/voltage relationship that varies with temperature. The variations may introduce uncertainties in the operation of the electronic circuits and may degrade the performance of the electronic circuits. Besides transistors and diodes, other parts of the electronic circuits may also be subject to operational variations in dependency of the temperature of the parts. Thus, there is a need for compensating the operation of these devices for temperature variations.
In many circuits voltage references and/or current references are used as a basic fundamental sub-circuit. Many of those current and voltage references are designed to be temperature independent, however, if they provide a well-defined temperature dependent current of voltage, their temperature dependency may be used to compensate for temperature effects in other parts of the circuitry.
The term Temperature Coefficient Factor (TCF) is introduced in this context and it is being used to refer to a slope of a current provided by a current source when the temperature varies. The unit of TCF is ppmK, which means, if the temperature changes with 1 K, the current provided by the current source varies with 1·10−6 A. A temperature compensation circuitry provides, preferably, a current with a well-defined TCF. In literature many examples of TCF circuits are provided which have such a well-defined TCF. In a number of applications, such as, for example, in radar applications, it is desired to have a current source which provides a current with a programmable TCF. Thus, it is required to have a specific TCF in response to a control signal. Traditional approaches are to manufacture a plurality of current sources with different TCF values and only switch on a specific one of the plurality of current sources in dependence of the control signal. Such a programmable TCF circuit requires a lot of circuitry to be manufactured on, for example, an integrated circuit and is, thus, relatively expensive.
In document U.S. Pat. No. 6,222,470 discloses a digitally programmable temperature coefficient factor (TCF) circuit. The circuit provides a reference current or a reference voltage which value varies with temperature in dependence of a programmable TCF. The reference voltage is obtained by providing the reference current to a resistor. The reference current is a summation of a first current and a second current. The first current has a programmable value and is a programmable portion of a first maximum current I1max which has a well-defined TCF. The second current has a programmable value and is a programmable portion of a second maximum current I1ma, which does not vary with temperature. The first current is generated by a first Digital-to-Analog-Converter circuit (DAC) which receives the first maximum current with the well-defined TCF and which receives a first digital signal. The first DAC divides the first maximum current in dependence of the first digital signal. The first digital signal may have a maximum value N1max, and the actual value N1, and the DAC divides the first maximum current by the ratio N1/N1max. Thus, the first current has the value (N1/N1max)-Imax,which implies that the TCF of the first current also varies with the value of N1. In this manner the TCF of the reference current provided by the circuit also varies with the value of N1. It is to be noted that the generation of second current is performed in an equal manner, with a second DAC. The value of the second current varies with a value N2, however, it has a TCF of about 0.
The cited patent U.S. Pat. No. 6,222,470 only discloses that the first current, which varies with a programmable TCF, is generated with a DAC. The patent remains silent about the specific implementation of this DAC. Based on the disclosure of document, it may be concluded that if the TCF of the first maximum current is positive, the circuitry of U.S. Pat. No. 6,222,470 can only generated reference currents with a positive first maximum current, which is in specific applications a major limitation. Further, although the implementation of the DAC's is not disclosed, it is expected that when they have to be implemented on silicon, they are a relatively large and, thus, expensive circuit.